It is known from U.S. Pat. No. 3,879,364 that polymerization in microsuspension of vinyl chloride in the presence of a seeding material, containing all the initiator required for polymerization, enjoys the advantages, in comparison with polymerization in simple microsuspension, of minimum initiator consumption, greater regularity of reaction, a lower degree of crusting in the reaction zone and a higher level of concentration of polymer in the latex formed.
It is also known from the copending application Ser. No. 914,352, filed June 12, 1978 (now U.S. Pat. No. 4,245,070) which application was filed as a continuation of application Ser. No. 679,654, filed Apr. 23, 1976, and entitled "Method of Polymerizing Vinyl Chloride in Seeded Microsuspension", that the use, in polymerization of vinyl chloride in microsuspension, of at least two seeding substances comprising different particle sizes, of which at least one of said substances contains all the initiator required for polymerization, makes it possible further to increase the concentration of polymer in the latex formed, to reduce its viscosity, to improve the granulometry of the particles and to produce substances having properties which make them particularly suitable for the preparation of plastisols.
Moreover, it is known from U.S. Pat. No. 4,091,197 that it is possible to reduce the amount of initiator used, increase the polymerization speed, and reduce the formation of crusting in the reaction zone, by activating the initiator, throughout the polymerisation operation, by an organo-soluble metal complex which is prepared beforehand or which is formed in situ by reaction between a water-soluble metal salt and a complexing agent.
Although the above-described processes permit vinyl chloride polymers to be produced by seeded microsuspension polymerisation on an industrial scale, it is essential to try to make improvements:
either in the reaction process in order to increase the profitability by reducing the polymerisation time and the amount of crusting formed, or to reduce the dangers of pollution by the monomer, with higher rates of conversion;
or in the substances to be produced, for example by reducing the amounts of water-soluble metal salts, the residues of which can cause degradation of the properties of the vinyl chloride polymer, in regard to heat stability and stability in respect of ultra-violet rays.
The improvement according to this invention makes it possible to achieve such improvements.